(1) Field of the Invention
The present invention relates to a high-strength zirconia type sintered body composed of zirconia-alumina (ZrO.sub.2 -Al.sub.2 O.sub.3), zirconia-spinel (ZrO.sub.2 -MgAl.sub.2 O.sub.4) or zirconia-mullite (ZrO.sub.2 -3Al.sub.2 O.sub.3.2SiO.sub.2), and a process for the preparation thereof.
(2) Description of the Prior Art
Recently, developments for utilizing a zirconia sintered body of the tetragonal system containing a small amount of Y.sub.2 O.sub.3 as a stabilizer (hereinafter referred to as "Y-PSZ" for brevity) as a mechanical construction material such as cutting tools, dies, nozzles and bearings have been vigorously carried out, because Y-PSZ exhibits a high strength and a high toughness. The sintered body of the present invention has extremely excellent strength characteristics promising a broadening of its use as a mechanical construction material and an increase of the reliability and durability of the sintered body when used as a mechanical construction material.
Y-PSZ is well-known as a high-strength sintered body, and it is considered that the high strength of this sintered body is due to the fact that zirconia of the tetragonal system contained in the sintered body is converted to the monoclinic system by the martensite type transition under stress. The inventors examined in detail the strength characteristics of Y-PSZ, and as the result, it has been found that the strength is not proportional to the quantity of zirconia of the tetragonal system but that the highest strength is obtained when a small amount of a cubic crystal is contained in addition to the tetragonal crystal.
From this finding, the inventors presumed that if alumina (Al.sub.2 O.sub.3), spinel (MgAl.sub.2 O.sub.4) or mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) having higher elastic modulus and strength is incorporated instead of part of zirconia of the cubic system, the strength will further be improved. A sintered body of the Y-PSZ-Al.sub.2 O.sub.3 type was already reported [Journal of Material Science, 17, 247-254 (1982)]. However, the reported bending strength of a hot-pressed sintered body is not so high as 1200 MPa and is lower than the bending strength of hot-pressed Y-PSZ (1300 to 1600 MPa), and the teaching of the prior art reference appears contradictory to the above presumption. However, the strength of the sintered body depends greatly on the starting material and the preparation process.